1. Field of the Invention
The invention relates to a microwave plasma processing system which is used for performing a processing such as etching, ashing, or thin film formation in a production of a semiconductor device.
2. Description of Related Art
In a microwave plasma processing system, a gas discharge is generated in a vacuum vessel under a reduced pressure or a low gas pressure by introducing microwaves into the vessel, thereby generating a plasma, and the surface of a substrate which is a sample is irradiated with the generated plasma so that a processing such as etching, or thin film formation is performed on the substrate. Since such a plasma processing system is essential in the production of a highly integrated semiconductor device and the like, research and development are intensively conducted on the system. Particularly, a microwave plasma processing system in which the generation of a plasma and the acceleration of ions in the generated plasma can independently be controlled is desired in the dry etching technique, and the burying technique for thin film formation.
FIG. 1 shows a conventional microwave plasma processing system 400. The system is aimed at independently controlling the generation of a plasma, and the acceleration of ions in the plasma. Specifically, the objectives of the system are to generate a stable bias voltage at the surface of a sample when a high frequency electric field is applied to a sample holder so as to optimize the energy of ions in a plasma, and to perpendicularly irradiate the surface of the sample with ions. In the figure, numeral 40 designates a reactor which is made of a metal such as aluminum, or stainless steel. The reactor 40 is airtightly partitioned by a microwave introduction window 45. The window consists of a refractory plate made of a material such as quartz glass, or Al.sub.2 O.sub.3 which allows microwaves to be transmitted therethrough, produces a small dielectric loss and has heat resistance.
In the upper portion of the reactor 40 which is partitioned by the microwave introduction window 45, disposed is a dielectric passage 44 which opposes the microwave introduction window 45 with being separated therefrom by a predetermined distance, and which has a size sufficient for covering the microwave introduction window 45. The dielectric passage 44 comprises a dielectric layer 44a which is made of a dielectric material producing a small dielectric loss, such as fluororesin, polystyrene, or polyethylene, and a metal plate 44b which is made of Al or the like and disposed on the upper face of the dielectric layer 44a. Microwaves are introduced from a microwave oscillator 47 into the dielectric passage 44 via a waveguide 46. The terminal of the dielectric passage 44 is sealed by the metal plate 44b.
In the lower portion which is partitioned by the microwave introduction window 45, a reaction chamber 41 is formed. A sample stage 42 having a sample holder 42a for holding a sample S which is to be processed is disposed inside the reaction chamber 41. A high frequency power source 43 for generating a bias voltage at the surface of the sample S is connected to the sample holder 42a. The sample holder 42a is provided with a suction mechanism for sucking the sample S such as an electrostatic chuck, and a cooling mechanism for cooling the sample S which uses a circulating coolant or the like. A metal plate 51 which is connected to the ground 52 via the reactor 40 is disposed in closely contact with the lower face of the microwave introduction window 45 which opposes the sample S. A large number of slits (or holes) 51a are formed in the metal plate 51 so that microwaves enter the reaction chamber 41. The metal plate 51 serves as an anode which is confronted with a cathode (the sample holder 42a) connected to the high frequency power source 43, thereby allowing a distinct bias voltage to be generated to the sample S mounted on the cathode.
The side wall of the reaction chamber 41 has a double-wall structure so that a cavity serving as a passage 50 for cooling water is formed inside the double-structured side wall. A cooling water inlet pipe 50a and a cooling water outlet pipe 50b are communicated with the passage 50. A gas supply pipe 48 through which gasses required for generating a plasma are supplied into the reaction chamber 41 is connected to the upper portion of the side wall. A gas discharge port 49 which is connected to an evacuating apparatus and through which the reaction chamber 41 is evacuated to attain a vacuum is coupled to the lower portion of the side wall.
Hereinafter, the case where a processing, for example, of an etching is to be performed on the surface of the sample S in the thus configured microwave plasma processing system 400 will be described.
First, evacuation is conducted through the gas discharge port 49 to set the interior of the reaction chamber 41 to have a predetermined pressure, and reactant gasses are then supplied through the gas supply pipe 48. Cooling water is supplied from the cooling water inlet pipe 50a to be circulated in the cavity 50, and then discharged into the cooling water outlet pipe 50b. Then the microwave oscillator 47 oscillates microwaves and the generated microwaves are introduced into the dielectric passage 44 via the waveguide 46 so that an electric field is generated under the dielectric passage 44. The generated electric field is transmitted through the microwave introduction window 45 and passes through the slits (or holes) 51a of the grounded metal plate 51 so that a plasma is generated in the reaction chamber 41. At the same time, in order to control the anisotropy and the acceleration energy of ions in the plasma, the high frequency power source 43 applies a high frequency electric field to the sample holder 42a on which the sample S is mounted. Then a stable bias voltage is generated at the surface of the sample S by the action of the grounded metal plate 51. The bias voltage causes the ions to be perpendicularly incident upon the sample S, and controls the energy of the ions incident upon the sample S, whereby the etching conditions are optimized.
In the microwave plasma processing system, the reactant gasses are introduced into the reaction chamber 41 with passing through the side wall of the reactor 40 (the reaction chamber 41) via the gas supply pipe 48, and are not directly supplied toward the sample S. Therefore, it is difficult to improve the utilization efficiency of reactant gasses, and the gasses flow toward the processed face of the sample S is ununiform. Consequently, it is difficult to improve the uniformities of processing, such as those of the etch rate, and the selection ratio of the SiO.sub.2 with respect to Si.
In order to enable the electric field generated from the dielectric passage 44 to enter the reaction chamber 41 via the microwave introduction window 45, the microwave transmission holes 51a having a slit-like or circular shape are formed in the metal plate 51. However, the shape of the microwave transmission holes 51a which is suitable for generating a high-density and uniform plasma is not revealed.
FIG. 2 is a schematic section view showing a conventional microwave plasma processing system which is proposed by the applicant in Japanese Patent Application Laid-Open No. 6-104098 (1994) with the objective of independently controlling the generation of a plasma and the acceleration of ions in the plasma.
In the system, a metal plate 53 is disposed so as to be in contact with the lower face of a microwave introduction window 45, or to be positioned at the midpoint between the microwave introduction window 45 and a sample holder 42a. A plurality of slits 53a which elongate in a direction perpendicular to the microwave traveling direction are formed in the metal plate 53. The metal plate 53 is grounded via a reactor 40 (as indicated by 52). The sample holder 42a for holding a sample S and a sample stage 54 on which the holder is mounted are disposed at a location opposing the metal plate 53 in a reaction chamber 41. The sample stage 54 is coupled with a driving mechanism so as to be vertically moved up and down. The other components similar to those of FIG. 1 are designated by the same reference numerals.
In the thus configured microwave plasma processing system, for example, a process of etching the surface of the sample S mounted on the sample holder 42a is performed in the following manner. First, the position of the sample S mounted on the sample holder 42a is adjusted by using the driving mechanism so that the sample is positioned at a predetermined height. Thereafter, evacuation is conducted through the discharge port 49. Reactant gasses are then introduced into the reaction chamber 41 via the gas supply pipe 48 and the interior of the reaction chamber 41 is set to have a desired pressure. Cooling water is supplied from the cooling water inlet pipe 50a to be circulated in the passage 50, and then discharged into the cooling water outlet pipe 50b. Then the microwave oscillator 47 oscillates microwaves and the generated microwaves are introduced into the dielectric passage 44 via the waveguide 46 so that an electric field is generated under the dielectric passage 44. The electric field is transmitted through the microwave introduction window 45 and passes through the slits 53a formed in the grounded metal plate 53 so that a plasma is generated in the reaction chamber 41. Then the high frequency power source 43 applies a high frequency electric field to the sample holder 42a so that a stable bias voltage is generated at the surface of the sample S by the action of the grounded metal plate 53. Then etching is performed while the bias voltage which is stably generated causes ions in the plasma to be perpendicularly incident upon the sample S and controls the energy of the ions.
In the microwave plasma processing system, the grounded metal plate 53 having the slits 53a is disposed so as to be in contact with the microwave introduction window 45 or to be positioned at the midpoint between the microwave introduction window 45 and the sample holder 42a. As a result, the ground potential with respect to the plasma is made stable so that the plasma potential is stabilized. When the high frequency electric field is applied to the sample holder 42a, therefore, a stable bias voltage can be generated at the surface of the sample S, the energy of the ions in the plasma can be optimized, and the ions can be perpendicularly irradiated on the surface of the samples. In the case where the ratio of the total area of the slits 53a to the outer form area of the metal plate 53 is small, a stable bias voltage can be generated. In such a case, however, there arises a problem that it is difficult to maintain a stable plasma discharge state based on a sufficient transmission of microwaves.
By contrast, in the case where the ratio of the total area of the slits 53a to the outer form area of the metal plate 53 is large, a stable plasma discharge can be generated. However, a problem is produced that it is difficult to generate the uniform and high bias voltage based on the distinct ground potential.